Local Earthquake Recordings

This example illustrates waveforms from a small local
earthquake. This event occurred in the city
of Berkeley, CA (just a few kilometers from
the Berkeley Seismographic Station!)
and was felt throughout the San
Francisco Bay area from Santa
Rosa to San Jose, and east to Concord. Some minor
damage was reported in the East Bay, including broken glass at one
Berkeley market.

Information from the UCB & USGS Menlo Park

Location:

4 km North West of Berkeley, CA

Date:

94/06/26

Origin Time (UTC):

08:42:50.7

Location:

37.907, -122.315

Depth (km):

4.2

Magnitude:

4.4 ML

A map illustrates the location of
this earthquake in the San Francisco Bay Area. The circle denotes
the location of the epicenter and the squares indicate the sites of
several BDSN seismometers. The solid lines indicate the location of
major active faults. Other lines connect the earthquake to the
stations shown in this example.

This event occurred on the Hayward Fault system and was well-recorded
by all stations of the BDSN.
This figure compares recordings
from 4 sites on the same plot. These seismograms are plotted with
amplitude as a function of time. These are raw waveforms and represent
the recorded ground velocity on the vertical component sampled at
20 Hz (the BHZ channel). The traces are ordered by distance and offset
by a fixed amount, with the closest station (BKS) at the bottom and
the most distant station (WDC) at the top. The traces are plotted
with absolute amplitude and the waveforms at BKS dominate the plot.
For each station, the distance to the earthquake in degrees is
labelled above the trace at the right edge of of the plot,
while the maximum amplitude is labeled below. This figure
illustrates the dramatic variation of amplitude with distance.

For comparison, this figure
shows the same seismograms, with each trace scaled by its own maximum
amplitude. This plots allows us to compare the waveforms among the
stations. The P-wave is the first arrival on these
vertical component seismograms and arrives later as the distance
between the earthquake and the station increases. The S-wave is
large amplitude phase, arriving after the P-wave. It is quite
clear at BKS, MHC, and CMB. At WDC, the S-wave arrival is less clear,
due to the emergence of the surface-wave train and local scattering.
The difference in the P and S-wave velocities is illustrated by
this figure, as the travel time between the P and S wave increases
with increasing distance.

The examples below include plots of the three components of ground
motion (BHZ - Vertical, BHN - North, BHE - East) recorded at 4 stations.
Each plot shows amplitude as a function of time, although the time
scale varies radically from station to station. The "absolute time" of
the plot is marked on the lower left, and the tic marks indicate
time relative to this absolute time.